In a current universal mobile telecommunications system (UMTS, Universal Mobile Telecommunications System), a 2×2 MIMO technology is adopted in a downlink. When a transmitting end adopts dual antennas to transmit a signal, where the two antennas are respectively configured to transmit a primary data stream, which is also called a primary transport block (Primary TB, Primary Transport Block), and a secondary data stream, which is also called a secondary transport block (Secondary TB, Secondary Transport Block). For a scheduled user, the primary transport block always exists, and whether the secondary transport block can be scheduled needs to be determined according to a channel condition. A rank of a 2×2 channel space is at most 2, which means at most two independent data streams can be transmitted, and simultaneous scheduling of a primary transport block and a secondary transport block is required. However, if there is a strong correlation between channels, only one independent data stream can be transmitted. That is, only the primary TB can be scheduled. Usually, information corresponding to a rank of an MIMO channel space is referred to as the number of layers (layers) in the MIMO channel space. Apparently, the number of supportable layers in a 2×2 MIMO system is 1 or 2.
In a telecommunications system, one transport block (TB, transport block) corresponds to a complete data block at a physical layer of a transmitting end, one codeword (CW, Codeword) corresponds to modulation and coding scheme (MCS, Modulation and Coding Scheme) information, and each codeword is coded and modulated separately. In a 2×2 downlink transmission scheme of a UMTS system, each TB is mapped to one codeword, and each codeword is mapped to one layer, and then TB transmission is performed. This means that each layer needs separate control signaling for ease of transmission. That is, each layer corresponds to a downlink MCS information and an uplink channel quality indicator (CQI, Channel Quality Indicator) and an acknowledgement/negative acknowledgement (ACK/NACK, Acknowledgement/Negative Acknowledgement) feedback.
With the development of UMTS, a 4×4 MIMO technology is introduced to downlink communications. This means that the number of supportable layers is increased to 4. There are three schemes: (1) one TB corresponds to one codeword, and one codeword corresponds to one layer, that is, 4 TB-4CodeWord-4Layer; (2) one or two TBs correspond to one codeword, and one codeword corresponds to one or two layers, that is, 4 TB-2CodeWord-4Layer; (3) one TB corresponds to one codeword, and one codeword corresponds to one or two layers, that is, 2 TB-2CodeWord4 Layer.
In the foregoing first and second schemes, if a CQI is fed back according to a channel quality of each layer, huge signaling overhead is caused. In order to decrease the signaling overhead, feeding back only one CQI for two layers can be considered. In this case, if two codewords are mapped to two separate layers, it means that a CQI needs to be fed back according to a layer with a lower quality, for example, a lower signal to noise ratio (SNR, Signal to Noise Ratio). Since the MCS during transmission is determined based on the CQI that is fed back, and if there is a great difference between the CQIs of the two layers, it means that a channel condition cannot be fully utilized, which results in lower throughput performance.
In addition, during MIMO transmission, an impact of inter-layer interference on signal transmission cannot be ignored. An advantage of the first and second schemes over the third scheme lies in that: in the first and second schemes, two TBs are used to replace one TB in the third scheme, and each TB corresponds to one cyclic redundancy check (CRC, Cyclic Redundancy Check), and as long as one of the two TBs is correctly transmitted, it is possible for a receiving end to adopt a successive interference cancellation (SIC, Successive Interference Cancellation) technology to demodulate the other TB, thereby increasing a probability that the two blocks are both correctly transmitted. However, the foregoing third scheme cannot make effective use of the SIC technology to improve transmission quality.
Therefore, it is necessary to improve the MIMO transmission technology in the prior art.